iPad vs. iPhone: what does 3G cost you?

With the announcement that the iPad would continue Apple's often-rocky relationship with AT&T to serve up wide-area data, we wanted to know: exactly what does an iPad cost you over the course of a couple years? Granted, the iPad doesn't support voice calling -- nor does it require a contract, unlike the iPhone -- but for anyone who uses an iPhone primarily for browsing and gaming, this could...